I've always been cautioned against ultrasonic cleaning of boards that have crystal oscillators, and indeed it's in most XO datasheets.
I've also heard that one shouldn't trim the leads of a through-hole XO before soldering it into the board, since the mechanical shock of the lead breaking can ring the whole package and similarly shake it apart. I'm curious if anyone here has seen that in practice!
I worked on a product that included pre-trimmed HC-49/S through-hole crystal oscillators, and not a single crystal failed. It was a low-volume product, but there were still probably tens too hundreds of thousands of them built.
When a batch with 20 MHz surface-mount crystals, in a package similar to the one in the article, were accidentally run through an ultrasonic cleaner, the failure rate was immediately noticeable, in the single-digit percent.
Leads of through-hole components are usually trimmed before assembly, on both manual and automated assembly lines, (e.g. https://www.youtube.com/watch?v=cjVY8lb0LG8) and I've never seen this prohibited in a datasheet, but ultrasonic cleaning is usually prohibited.
> I've also heard that one shouldn't trim the leads of a through-hole XO before soldering it into the board, since the mechanical shock of the lead breaking can ring the whole package and similarly shake it apart. I'm curious if anyone here has seen that in practice!
I’ve never put a through hole crystal into production so I can’t say anything about this conjecture.
However the larger surface mount crystals are not hard to hand solder if you get a package with side wettable flanks and make the pads reasonably large. It’s something I’d recommend considering.
Sometimes, the bigger physical size of through-hole crystals gives them a higher Q. I, too, prefer surface-mount everything but have been defeated on that sadly ;(
I went down this rabbit hole a few years ago, and couldn't find an actionable answer on if this is OK or not. Sounded like "No, you shouldn't", but almost every PCB I've designed (or used?) has at least one, and I know ultrasonic cleaning is a thing, so I'm not sure how to reconcile these.
There is no single answer. It depends on the exact components, their sensitivities, frequencies and energies used, and how much failure risk are you willing to take.
Rule of thumb: one simple xtal per board in small manufacturing runs (4 digits or less) means you're fine.
The larger your manufacturing runs are, and the more sensitive components you have on your boards, the more careful you want to be. Components can easily make the difference between 0.2% failure rate and 2% failure rate, and that 2% failure rate bites when you push units by hundreds of thousands.
Of course, there's always a chance of you getting a perfect match of the exact intensity and frequency used on a given manufacturing line, which you didn't know, with what happens to kill your specific components at a disproportionate rate, which you also didn't know. But it's a pretty low chance. Feeling lucky?
Because yes, it's not actually worth the engineering/support effort for you, your manufacturer and your part vendor to actually put the thinking cap on and characterize all of that shit for a typical low volume run. So luck it is.
On the other hand I heard that one shouldn't trim leads after soldering as it might crack solder joints...
Depends on the quality of the solder joint. Poor quality solder joints do not survive mechanical shock. If you are fighting a GND pin that sinks a lot of heat, using leadfree solder and you aren't that skilled...don't trim that lead flush with the PCB. Otherwise, if you are sure that the solder has wicked into the hole, trim away.
So if someone is telling you not to trim the lead...I'll let you draw your own conclusion.
This depends how close to the solder joint (or to board) you are trimming. If you're already cutting solder together with the component lead then it's too close and can affect the quality. I'm sure the NASA soldering manuals show this in great detail.
Oh, that's a good one, I can see how that would put a lot of g's on the package. I think this will be a factor depending on the weight of the total assembly. If that weight is significant it will dampen the shockwave.